Postulates of Dalton's atomic theory

John Dalton, a British school teacher, published his theory about atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.

Main assumptions or postulates of Dalton

• All matter consists of indivisible particles called atoms.

• Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.

• Atoms cannot be created or destroyed.

• Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.

• Atoms of same element can combine in more than one ratio to form two or more compounds.

• Atoms are the smallest unit of matter that can take part in a chemical reaction.

Drawbacks of Dalton's atomic theory of matter

• The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.

• According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.

• Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.

• According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C₁₂H₂₂O₁₁.

• The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.

Merits of Dalton's atomic theory

• It has enabled us to explain the laws of chemical combination.

• Dalton was the first person to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).